Conventionally, spark ignition engines have been developed to operate on approximations of the so-called Otto cycle wherein a combustible air-fuel mixture is compressed and burned in an approximately constant volume process occurring near the smallest volume of the combustion chamber. Operation on this cycle, while quite efficient, leads to high rates of pressure rise and high combustion temperatures which may require control of combustion rates through fuel additives to reduce problems of detonation or autoignition, as well as resulting in substantial exhaust emissions.
In recent years various devices and methods for modifying the combustion process have been utilized to obtain reductions in exhaust emissions. Such changes have included retarding spark timing, recirculating a portion of the exhaust gases and other methods for altering combustion. Although such methods have met with some success, they have also in general resulted in reduced efficiency of engine operation, which resultant increases in gasoline consumption for the vehicles powered by such engines.